185 patients scheduled for metal-on-metal (MoM) total hip arthroplasty (THA) were randomized to either large-head or small-head articulation. Main outcomes of concern in this study included whole blood cobalt and chromium ion concentrations and the incidence of pseudotumor development on MRI over a 5-year follow-up. Cobalt levels were significantly lower in the large-head group compared to the small-head group at 36, 48, and 60 months postoperatively, while chromium levels were significantly lower in the large-head group compared to the small-head group at 12 and 48 months postoperatively. The incidence of moderate pseudotumor development was significantly lower in the large-head group compared to the small-head group.